Non-blocked phosphorescent OLEDs

ABSTRACT

An organic light emitting diode (OLED) architecture in which efficient operation is achieved without requiring a blocking layer by locating the recombination zone close to the hole transport side of the emissive layer. Aryl-based hosts and Ir-based dopants with suitable concentrations result in an efficient phosphorescent OLED structure. Previously, blocking layer utilization in phosphorescent OLED architectures was considered essential to avoid exciton and hole leakage from the emissive layer, and thus keep the recombination zone inside the emissive layer to provide high device efficiency and a pure emission spectrum.

The claimed invention was made by, on behalf of, and/or in connectionwith one or more of the following parties to a joint universitycorporation research agreement: Princeton University, The University ofSouthern California, and the Universal Display Corporation. Theagreement was in effect on and before the date the claimed invention wasmade, and the claimed invention was made as a result of activitiesundertaken within the scope of the agreement.

FIELD OF THE INVENTION

The present invention relates to organic light emitting devices (OLEDs),and more specifically to OLEDs which may omit hole- and/orexciton-blocking layers.

BACKGROUND

Opto-electronic devices that make use of organic materials are becomingincreasingly desirable for a number of reasons. Many of the materialsused to make such devices are relatively inexpensive, so organicopto-electronic devices have the potential for cost advantages overinorganic devices. In addition, the inherent properties of organicmaterials, such as their flexibility, may make them well suited forparticular applications such as fabrication on a flexible substrate.Examples of organic opto-electronic devices include organic lightemitting devices (OLEDs), organic phototransistors, organic photovoltaiccells, and organic photodetectors. For OLEDs, the organic materials mayhave performance advantages over conventional materials. For example,the wavelength at which an organic emissive layer emits light maygenerally be readily tuned with appropriate dopants.

As used herein, doping percentages are quoted by weight percent.

As used herein, the term “organic” includes polymeric materials as wellas small molecule organic materials that may be used to fabricateorganic opto-electronic devices. “Small molecule” refers to any organicmaterial that is not a polymer, and “small molecules” may actually bequite large. Small molecules may include repeat units in somecircumstances. For example, using a long chain alkyl group as asubstituent does not remove a molecule from the “small molecule” class.Small molecules may also be incorporated into polymers, for example as apendent group on a polymer backbone or as a part of the backbone. Smallmolecules may also serve as the core moiety of a dendrimer, whichconsists of a series of chemical shells built on the core moiety. Thecore moiety of a dendrimer may be an fluorescent or phosphorescent smallmolecule emitter. A dendrimer may be a “small molecule,” and it isbelieved that all dendrimers currently used in the field of OLEDs aresmall molecules. In general, a small molecule has a well-definedchemical formula with a single molecular weight, whereas a polymer has achemical formula and a molecular weight that may vary from molecule tomolecule.

OLEDs make use of thin organic films that emit light when voltage isapplied across the device. OLEDs are becoming an increasinglyinteresting technology for use in applications such as flat paneldisplays, illumination, and backlighting. Several OLED materials andconfigurations are described in U.S. Pat. Nos. 5,844,363, 6,303,238, and5,707,745, which are incorporated herein by reference for the materialsand configurations disclosed therein.

OLED devices are generally (but not always) intended to emit lightthrough at least one of the electrodes, and one or more transparentelectrodes may be useful in an organic opto-electronic devices. Forexample, a transparent electrode material, such as indium tin oxide(ITO), may be used as the bottom electrode. A transparent top electrodemay also be used, such as disclosed in U.S. Pat. Nos. 5,703,436 and5,707,745, which are incorporated by reference for their disclosurerelated to transparent electrodes. For a device intended to emit lightonly through the bottom electrode, the top electrode does not need to betransparent, and may be comprised of a thick and reflective metal layerhaving a high electrical conductivity. Similarly, for a device intendedto emit light only through the top electrode, the bottom electrode maybe opaque and/or reflective. Where an electrode does not need to betransparent, using a thicker layer may provide better conductivity, andusing a reflective electrode may increase the amount of light emittedthrough the other electrode, by reflecting light back towards thetransparent electrode. Fully transparent devices may also be fabricated,where both electrodes are transparent. Side emitting OLEDs may also befabricated, and one or both electrodes may be opaque or reflective insuch devices.

As used herein, “top” means furthest away from the substrate, while“bottom” means closest to the substrate. For example, for a devicehaving two electrodes, the bottom electrode is the electrode closest tothe substrate, and is generally the first electrode fabricated. Thebottom electrode has two surfaces, a bottom surface closest to thesubstrate, and a top surface further away from the substrate. Where afirst layer is described as “disposed over” a second layer, the firstlayer is disposed further away from substrate. There may be other layersbetween the first and second layer, unless it is specified that thefirst layer is “in physical contact with” the second layer. For example,a cathode may be described as “disposed over” an anode, even thoughthere are various organic layers in between.

As used herein, “solution processible” means capable of being dissolved,dispersed, or transported in and/or deposited from a liquid medium,either in solution or suspension form.

As used herein, and as would be generally understood by one skilled inthe art, a first “Highest Occupied Molecular Orbital” (HOMO) or “LowestUnoccupied Molecular Orbital” (LUMO) energy level is “greater than” or“higher than” a second HOMO or LUMO energy level if the first energylevel is closer to the vacuum energy level. Since ionization potentials(IP) are measured as a negative energy relative to a vacuum level, ahigher HOMO energy level corresponds to an IP having a smaller absolutevalue (an IP that is less negative). Similarly, a higher LUMO energylevel corresponds to an electron affinity (EA) having a smaller absolutevalue (an EA that is less negative). On a conventional energy leveldiagram, with the vacuum level at the top, the LUMO energy level of amaterial is higher than the HOMO energy level of the same material. A“higher” HOMO or LUMO energy level appears closer to the top of such adiagram than a “lower” HOMO or LUMO energy level.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an organic light emitting device having separate electrontransport, hole transport, and emissive layers, as well as other layers.

FIG. 2 shows an inverted organic light emitting device that does nothave a separate electron transport layer.

FIG. 3 shows an example energy band diagram for an OLED with a blockinglayer.

FIG. 4 shows the normalized electroluminescent spectra of anIr(5-Phppy)₃ (12%) phosphorescent OLED with 2 nm 0.5% rubrene probesinserted into the emissive layer close to the hole transport layer, inthe middle of the emissive layer, and close to the electron transportlayer.

FIG. 5 shows the normalized electroluminescent spectra of anIr(3′-Meppy)₃ (6%) phosphorescent OLED with 2 nm 0.5% rubrene probesinserted into the emissive layer close to the hole transport layer, inthe middle of the emissive layer, and close to the electron transportlayer.

FIG. 6 shows an OLED structure used to compare the electron transportingproperties of 2,7-DCP and CBP, with a layer of 2,7-DCP or CBP placedbetween an Alq₃ electron transport layer and a NPD hole transport layer.

FIG. 7 shows the normalized EL spectra of the devices of FIG. 6.

FIG. 8 shows a structure similar to that in FIG. 6 used to compare theelectron transporting properties of 2,7-DCP and CBP, but in which thelayer of 2,7-DCP or CBP is inserted into the Alq₃ electron transportlayer.

FIG. 9 shows the current density versus operating voltage for thedevices of FIG. 8.

FIG. 10 shows the external quantum efficiency versus current density ofthe devices in FIG. 8.

FIG. 11 is a schematic energy level diagram of a phosphorescent OLEDwithout blocking layers.

FIG. 12 is a structure without a blocking layer used to performexperimental comparisons between CBP as a host and 2,7-DCP as a host.

FIG. 13 illustrates the normalized electroluminescence spectra at 10mA/cm² for the experimental comparisons demonstrated with the structureof FIG. 12.

FIG. 14 illustrates the luminous efficiency versus luminance for theexperimental comparisons demonstrated with the structure of FIG. 12.

FIG. 15 illustrates the power efficiency versus the luminance for theexperimental comparisons demonstrated with the structure of FIG. 12.

FIG. 16 illustrates the current density versus voltage for theexperimental comparisons demonstrated with the structure of FIG. 12.

FIG. 17 illustrates the luminance versus voltage for the experimentalcomparisons demonstrated with the structure of FIG. 12.

FIG. 18 shows the lifetime at room temperature with 40 mA/cm² fornon-blocked green structure demonstrated in examples 1 and 3 with FIG.12.

FIG. 19 shows the lifetime at room temperature with 1000 cd/m² fornon-blocked green structure demonstrated in example 1 with FIG. 12.

DETAILED DESCRIPTION

Generally, an OLED comprises at least one organic layer disposed betweenand electrically connected to an anode and a cathode. When a current isapplied, the anode injects holes and the cathode injects electrons intothe organic layer(s). The injected holes and electrons each migratetoward the oppositely charged electrode. When an electron and holelocalize on the same molecule, an “exciton,” which is a localizedelectron-hole pair having an excited energy state, is formed. Light isemitted when the exciton relaxes via a photoemissive mechanism. In somecases, the exciton may be localized on an excimer or an exciplex.Non-radiative mechanisms, such as thermal relaxation, may also occur,but are generally considered undesirable.

The initial OLEDs used emissive molecules that emitted light from theirsinglet states (“fluorescence”) as disclosed, for example, in U.S. Pat.No. 4,769,292, which is incorporated by reference for its explanation ofOLEDs with light emission from singlet states. Fluorescent emissiongenerally occurs in a time frame of less than 10 nanoseconds.

More recently, OLEDs having emissive materials that emit light fromtriplet states (“phosphorescence”) have been demonstrated. Baldo et al.,“Highly Efficient Phosphorescent Emission from OrganicElectroluminescent Devices,” Nature, vol. 395, 151-154, 1998;(“Baldo-I”) and Baldo et al., “Very high-efficiency green organiclight-emitting devices based on electrophosphorescence,” Appl. Phys.Lett., vol. 75, No. 3, 4-6 (1999) (“Baldo-II”), which are incorporatedby reference for their explanation of OLEDs with light emission fromtriplet states. Phosphorescence may be referred to as a “forbidden”transition because the transition requires a change in spin states, andquantum mechanics indicates that such a transition is not favored. As aresult, phosphorescence generally occurs in a time frame exceeding atleast 10 nanoseconds, and typically greater than 100 nanoseconds. If thenatural radiative lifetime of phosphorescence is too long, triplets maydecay by a non-radiative mechanism, such that no light is emitted.Organic phosphorescence is also often observed in molecules containingheteroatoms with unshared pairs of electrons at very low temperatures.2,2′-bipyridine is such a molecule. Non-radiative decay mechanisms aretypically temperature dependent, such that an organic material thatexhibits phosphorescence at liquid nitrogen temperatures typically doesnot exhibit phosphorescence at room temperature. But, as demonstrated byBaldo, this problem may be addressed by selecting phosphorescentcompounds that do phosphoresce at room temperature. Representativeemissive layers include doped or un-doped phosphorescent organometallicmaterials such as disclosed in U.S. Pat. Nos. 6,303,238 and 6,310,360;U.S. Patent Application Publication Nos. 2002-0034656; 2002-0182441;2003-0072964; and WO-02/074015.

Generally, the excitons in an OLED are believed to be created in a ratioof about 3:1, i.e., approximately 75% triplets and 25% singlets. See,Adachi et al., “Nearly 100% Internal Phosphorescent Efficiency In AnOrganic Light Emitting Device,” J. Appl. Phys., 90, 5048 (2001), whichis incorporated by reference for its explanation of the excitonformation process. In many cases, singlet excitons may readily transfertheir energy to triplet excited states via “intersystem crossing,”whereas triplet excitons may not readily transfer their energy tosinglet excited states. As a result, 100% internal quantum efficiency istheoretically possible with phosphorescent OLEDs. In a fluorescentdevice, the energy of triplet excitons is generally lost toradiationless decay processes that heat-up the device, resulting in muchlower internal quantum efficiencies. OLEDs utilizing phosphorescentmaterials that emit from triplet excited states are disclosed, forexample, in U.S. Pat. No. 6,303,238, which is incorporated by referencefor the structures and materials used to emit from triplet excitedstates.

Phosphorescence may be preceded by a transition from a triplet excitedstate to an intermediate non-triplet state from which the emissive decayoccurs. For example, organic molecules coordinated to lanthanideelements often phosphoresce from excited states localized on thelanthanide metal. However, such materials do not phosphoresce directlyfrom a triplet excited state but instead emit from an atomic excitedstate centered on the lanthanide metal ion. The europium diketonatecomplexes illustrate one group of these types of species.

Phosphorescence from triplets can be enhanced over fluorescence byconfining, preferably through bonding, the organic moiety in closeproximity to an atom of high atomic number. This phenomenon, called theheavy atom effect, is created by a mechanism known as spin-orbitcoupling. Such a phosphorescent transition may be observed from anexcited metal-to-ligand charge transfer (MLCT) state of anorganometallic molecule such as tris(2-phenylpyridine)iridium(III).

As used herein, the term “triplet energy” refers to an energycorresponding to the highest energy feature discernable in thephosphorescence spectrum of a given material. The highest energy featureis not necessarily the peak having the greatest intensity in thephosphorescence spectrum, and could, for example, be a local maximum ofa clear shoulder on the high energy side of such a peak.

The term “organometallic” as used herein is as generally understood byone of ordinary skill in the art and as given, for example, in“Inorganic Chemistry” (2nd Edition) by Gary L. Miessler and Donald A.Tarr, Prentice Hall (1998). Thus, the term organometallic refers tocompounds which have an organic group bonded to a metal through acarbon-metal bond. This class does not include per se coordinationcompounds, which are substances having only donor bonds fromheteroatoms, such as metal complexes of amines, halides, pseudohalides(CN, etc.), and the like. In practice organometallic compounds generallycomprise, in addition to one or more carbon-metal bonds to an organicspecies, one or more donor bonds from a heteroatom. The carbon-metalbond to an organic species refers to a direct bond between a metal and acarbon atom of an organic group, such as phenyl, alkyl, alkenyl, etc.,but does not refer to a metal bond to an “inorganic carbon,” such as thecarbon of CN or CO.

FIG. 1 shows an example organic light emitting device 100. The figuresare not necessarily drawn to scale. Device 100 may include a substrate110, an anode 115, a hole injection layer 120, a hole transport layer125, an electron blocking layer 130, an emissive layer 135, a holeblocking layer 140, an electron transport layer 145, an electroninjection layer 150, a protective layer 155, and a cathode 160. Cathode160 is a compound cathode having a first conductive layer 162 and asecond conductive layer 164. Device 100 may be fabricated by depositingthe layers described, in order.

Substrate 110 may be any suitable substrate that provides desiredstructural properties. Substrate 110 may be flexible or rigid. Substrate110 may be transparent, translucent or opaque. Plastic and glass areexamples of preferred rigid substrate materials. Plastic and metal foilsare examples of preferred flexible substrate materials. Substrate 110may be a semiconductor material in order to facilitate the fabricationof circuitry. For example, substrate 110 may be a silicon wafer uponwhich circuits are fabricated, capable of controlling OLEDs subsequentlydeposited on the substrate. Other substrates may be used. The materialand thickness of substrate 110 may be chosen to obtain desiredstructural and optical properties.

Anode 115 may be any suitable anode that is sufficiently conductive totransport holes to the organic layers. The material of anode 115preferably has a work function higher than about 4 eV (a “high workfunction material”). Preferred anode materials include conductive metaloxides, such as indium tin oxide (ITO) and indium zinc oxide (IZO),aluminum zinc oxide (AlZnO), and metals. Anode 115 (and substrate 110)may be sufficiently transparent to create a bottom-emitting device. Apreferred transparent substrate and anode combination is commerciallyavailable ITO (anode) deposited on glass or plastic (substrate). Aflexible and transparent substrate-anode combination is disclosed inU.S. Pat. Nos. 5,844,363 and 6,602,540 B2, which are incorporated byreference for their disclosure regarding flexible and transparentsubstrate-anode combinations. Anode 115 may be opaque and/or reflective.A reflective anode 115 may be preferred for some top-emitting devices,to increase the amount of light emitted from the top of the device. Thematerial and thickness of anode 115 may be chosen to obtain desiredconductive and optical properties. Where anode 115 is transparent, theremay be a range of thickness for a particular material that is thickenough to provide the desired conductivity, yet thin enough to providethe desired degree of transparency. Other anode materials and structuresmay be used.

Hole transport layer (HTL) 125 may include a material capable oftransporting holes. Hole transport layer 130 may be intrinsic (undoped),or doped. Doping may be used to enhance conductivity. α-NPD and TPD areexamples of intrinsic hole transport layers. An example of a p-dopedhole transport layer is m-MTDATA doped with F₄-TCNQ at a molar ratio of50:1, as disclosed in United States Patent Application Publication No.2002-0071963 A1 to Forrest et al., which is incorporated by reference inits entirety. Other hole transport layers may be used.

Emissive layer 135 may include an organic material capable of emittinglight when a current is passed between anode 115 and cathode 160.Preferably, emissive layer 135 contains a phosphorescent emissivematerial, although fluorescent emissive materials may also be used.Phosphorescent materials are preferred because of the higher luminescentefficiencies associated with such materials. Emissive layer 135 may alsocomprise a host material capable of transporting electrons and/or holes,doped with an emissive material that may trap electrons, holes, and/orexcitons, such that excitons relax from the emissive material via aphotoemissive mechanism. Emissive layer 135 may comprise a singlematerial that combines transport and emissive properties. Whether theemissive material is a dopant or a major constituent, emissive layer 135may comprise other materials, such as dopants that tune the emission ofthe emissive material. Emissive layer 135 may include a plurality ofemissive materials capable of, in combination, emitting a desiredspectrum of light. Examples of phosphorescent emissive materials includeIr(ppy)₃. Examples of fluorescent emissive materials include DCM andDMQA. Examples of host materials include Alq₃, CBP and mCP. Examples ofemissive and host materials are disclosed in U.S. Pat. No. 6,303,238 toThompson et al., which is incorporated by reference for its disclosureregarding emissive and host materials. Emissive material may be includedin emissive layer 135 in a number of ways. For example, an emissivesmall molecule may be incorporated into a polymer. This may beaccomplished by several ways: by doping the small molecule into thepolymer either as a separate and distinct molecular species; or byincorporating the small molecule into the backbone of the polymer, so asto form a co-polymer; or by bonding the small molecule as a pendantgroup on the polymer. Other emissive layer materials and structures maybe used. For example, a small molecule emissive material may be presentas the core of a dendrimer.

Many useful emissive materials include one or more ligands bound to ametal center. A ligand may be referred to as “photoactive” if itcontributes directly to the photoactive properties of an organometallicemissive material. A “photoactive” ligand may provide, in conjunctionwith a metal, the energy levels from which and to which an electronmoves when a photon is emitted. Other ligands may be referred to as“ancillary.” Ancillary ligands may modify the photoactive properties ofthe molecule, for example by shifting the energy levels of a photoactiveligand, but ancillary ligands do not directly provide the energy levelsinvolved in light emission. A ligand that is photoactive in one moleculemay be ancillary in another. These definitions of photoactive andancillary are intended as non-limiting theories.

Electron transport layer (ETL) 145 may include a material capable oftransporting electrons. Electron transport layer 145 may be intrinsic(undoped), or doped. Doping may be used to enhance conductivity. Alq₃ isan example of an intrinsic electron transport layer. An example of ann-doped electron transport layer is BPhen doped with Li at a molar ratioof 1:1, as disclosed in United States Patent Application Publication No.2002-0071963 A1 to Forrest et al., which is incorporated by reference inits entirety. Other electron transport layers may be used.

The charge carrying component of the electron transport layer may beselected such that electrons can be efficiently injected from thecathode into the LUMO (Lowest Unoccupied Molecular Orbital) energy levelof the electron transport layer. The “charge carrying component” is thematerial responsible for the LUMO energy level that actually transportselectrons. This component may be the base material, or it may be adopant. The LUMO energy level of an organic material may be generallycharacterized by the electron affinity of that material and the relativeelectron injection efficiency of a cathode may be generallycharacterized in terms of the work function of the cathode material.This means that the preferred properties of an electron transport layerand the adjacent cathode may be specified in terms of the electronaffinity of the charge carrying component of the ETL and the workfunction of the cathode material. In particular, so as to achieve highelectron injection efficiency, the work function of the cathode materialis preferably not greater than the electron affinity of the chargecarrying component of the electron transport layer by more than about0.75 eV, more preferably, by not more than about 0.5 eV. Similarconsiderations apply to any layer into which electrons are beinginjected.

Cathode 160 may be any suitable material or combination of materialsknown to the art, such that cathode 160 is capable of conductingelectrons and injecting them into the organic layers of device 100.Cathode 160 may be transparent or opaque, and may be reflective. Metalsand metal oxides are examples of suitable cathode materials. Cathode 160may be a single layer, or may have a co-pound structure. FIG. 1 shows acompound cathode 160 having a thin metal layer 162 and a thickerconductive metal oxide layer 164. In a compound cathode, preferredmaterials for the thicker layer 164 include ITO, IZO, and othermaterials known to the art. U.S. Pat. Nos. 5,703,436, 5,707,745,6,548,956 B2 and 6,576,134 B2, are incorporated by reference for theirdisclosure of cathodes including compound cathodes having a thin layerof metal (such as Mg:Ag) with an overlying transparent,electrically-conductive layer (such as a sputter-deposited ITO layer).The part of cathode 160 that is in contact with the underlying organiclayer, whether it is a single layer cathode 160, the thin metal layer162 of a compound cathode, or some other part, is preferably made of amaterial having a work function lower than about 4 eV (a “low workfunction material”). Other cathode materials and structures may be used.

Blocking layers may be used to reduce the number of charge carriers(electrons or holes) and/or excitons that leave the emissive layer. Anelectron blocking layer (EBL) 130 may be disposed between emissive layer135 and the hole transport layer 125, to block electrons from leavingemissive layer 135 in the direction of hole transport layer 125.Similarly, a hole blocking layer (HBL) 140 may be disposed betweenemissive layer 135 and electron transport layer 145, to block holes fromleaving emissive layer 135 in the direction of electron transport layer145. Blocking layers may also be used to block excitons from diffusingout of the emissive layer. The theory and use of blocking layers isdescribed in more detail in U.S. Pat. No. 6,097,147 and United StatesPatent Application Publication No. 2002-0071963 A1 to Forrest et al.,which are incorporated by reference for their disclosure regarding thetheory and use of blocking layers.

As used herein, and as would be understood by one skilled in the art,the term “blocking layer” means that the layer provides a barrier thatsignificantly inhibits transport of charge carriers and/or excitonsthrough the device, without suggesting that the layer necessarilycompletely blocks the charge carriers and/or excitons. The presence ofsuch a blocking layer in a device may result in substantially higherefficiencies as compared to a similar device lacking a blocking layer.Also, a blocking layer may be used to confine emission to a desiredregion of an OLED.

Generally, injection layers are comprised of a material that may improvethe injection of charge carriers from one layer, such as an electrode oran organic layer, into an adjacent organic layer. Injection layers mayalso perform a charge transport function. In device 100, hole injectionlayer 120 may be any layer that improves the injection of holes fromanode 115 into hole transport layer 125. CuPc is an example of amaterial that may be used as a hole injection layer from an ITO anode115, and other anodes. In device 100, electron injection layer 150 maybe any layer that improves the injection of electrons into electrontransport layer 145. LiF/Al is an example of a material that may be usedas an electron injection layer into an electron transport layer from anadjacent layer. Other materials or combinations of materials may be usedfor injection layers. Depending upon the configuration of a particulardevice, injection layers may be disposed at locations different thanthose shown in device 100. More examples of injection layers areprovided in U.S. patent application Ser. No. 09/931,948 to Lu et al.,which is incorporated by reference in its entirety. A hole injectionlayer may comprise a solution deposited material, such as a spin-coatedpolymer, e.g., PEDOT:PSS, or it may be a vapor deposited small moleculematerial, e.g., CuPc or MTDATA.

A hole injection layer (HIL) may planarize or wet the anode surface soas to provide efficient hole injection from the anode into the holeinjecting material. A hole injection layer may also have a chargecarrying component having HOMO (Highest Occupied Molecular Orbital)energy levels that favorably match up, as defined by theirherein-described relative ionization potential (IP) energies, with theadjacent anode layer on one side of the HIL and the hole transportinglayer on the opposite side of the HIL. The “charge carrying component”is the material responsible for the HOMO energy level that actuallytransports holes. This component may be the base material of the HIL, orit may be a dopant. Using a doped HIL allows the dopant to be selectedfor its electrical properties, and the host to be selected formorphological properties such as wetting, flexibility, toughness, etc.Preferred properties for the HIL material are such that holes can beefficiently injected from the anode into the HIL material. Inparticular, the charge carrying component of the HIL preferably has anIP not more than about 0.7 eV lower than the IP of the anode material.More preferably, the charge carrying component has an IP not more thanabout 0.5 eV lower than the anode material. Similar considerations applyto any layer into which holes are being injected. HIL materials arefurther distinguished from conventional hole transporting materials thatare typically used in the hole transporting layer of an OLED in thatsuch HIL materials may have a hole conductivity that is substantiallyless than the hole conductivity of conventional hole transportingmaterials. The thickness of the HIL of the present invention may bethick enough to help planarize or wet the surface of the anode layer.For example, an HIL thickness of as little as 10 nm may be acceptablefor a very smooth anode surface. However, since anode surfaces tend tobe very rough, a thickness for the HIL of up to 50 nm may be desired insome cases.

Organometallic complexes as hole injection materials are hereindemonstrated to be useful alternatives to those commonly used such asmetal phthalocyanines and arylamines. Ir(ppy)₃ type complexes such asIr(3′-Meppy)₃ are shown as examples. Other complexes such as andIr(5-Phppy)₃ and Ir(1-phenylisoquinoline)₃ can also be used. A widerange of Ir organometallic complexes such as Ir(2-phenylimidazole)₃,Ir(2-phenylbenzimidazole)₃, Ir(N-alkyl-N′-arylimidazole)₃ may be usedbecause of the facile and reversible oxidation of the Ir(III) metalcenter. Further representative examples include organometallic complexeshaving substituted or unsubstituted ligands such as phenylpyridines,phenylimidazoles, and phenylquinolines or substituted or unsubstitutedcarbene ligands. Organometallic complexes with other metal such asCo(III), Fe(II), Ru(II), Os(II), etc may also be used. In organometalliccomplexes, the oxidation potential can be tuned by the choice of metalsand the electron donating/withdrawing nature of the ligands. Otherphysical properties such as sublimation temperature, glass transitiontemperature, solubility, etc, can also be tuned by the chemicalmodification of the ligands.

A protective layer may be used to protect underlying layers duringsubsequent fabrication processes. For example, the processes used tofabricate metal or metal oxide top electrodes may damage organic layers,and a protective layer may be used to reduce or eliminate such damage.In device 100, protective layer 155 may reduce damage to underlyingorganic layers during the fabrication of cathode 160. Preferably, aprotective layer has a high carrier mobility for the type of carrierthat it transports (electrons in device 100), such that it does notsignificantly increase the operating voltage of device 100. CuPc, BCP,and various metal phthalocyanines are examples of materials that may beused in protective layers. Other materials or combinations of materialsmay be used. The thickness of protective layer 155 is preferably thickenough that there is little or no damage to underlying layers due tofabrication processes that occur after organic protective layer 160 isdeposited, yet not so thick as to significantly increase the operatingvoltage of device 100. Protective layer 155 may be doped to increase itsconductivity. For example, a CuPc or BCP protective layer 160 may bedoped with Li. A more detailed description of protective layers may befound in U.S. patent application Ser. No. 09/931,948 to Lu et al., whichis incorporated by reference for the description of protective layers.

FIG. 2 shows an example inverted OLED 200. The device includes asubstrate 210, an cathode 215, an emissive layer 220, a hole transportlayer 225, and an anode 230. Device 200 may be fabricated by depositingthe layers described, in order. Because the most common OLEDconfiguration has a cathode disposed over the anode, and device 200 hascathode 215 disposed under anode 230, device 200 may be referred to asan “inverted” OLED. Materials similar to those described with respect todevice 100 may be used in the corresponding layers of device 200. FIG. 2provides one example of how some layers may be omitted from thestructure of device 100.

Previously, blocking layer utilization in phosphorescent OLEDarchitectures was considered important to avoid exciton and hole leakagefrom the emissive layer, and thus keep the recombination zone inside theemissive layer to provide high device efficiency and a pure emissionspectrum. FIG. 3 shows an example energy band diagram for an OLED with ablocking layer. The ability to achieve efficient light generation whileomitting the blocking layer would reduce manufacturing costs. However,light generation with high efficiency and lifetime has generally notbeen achieved without the inclusion of a blocking layer, particularlyfor wavelengths corresponding to green and shorter.

An example phosphorescent OLED structure consists of anode/holeinjection layer/hole transport layer/Host:Dopant/second electrontransport layer/first electron transport layer/cathode. Alq₃ is mostcommonly used as the first electron transport layer (ETL1) because ofits good electron transport properties and stability. The secondelectron transport layer (ETL2) may be used to facilitate injectingelectrons into the emissive layer (EML) and blocking holes from the EMLto achieve good efficiency. This may be particularly important innon-red phosphorescent OLEDs because in non-red phosphorescent OLEDs,the hosts must have an energy gap (Eg) that is not too low which impliesHOMO and/or LUMO are not as easily accessible as in the case for redphosphorescent OLEDs in which lower gap hosts can be used. For example,a typical green phosphorescent OLED structure isanode/CuPc/α-NPD/CBP:Ir(ppy)₃/ETL2/Alq₃/cathode where ETL1 is BCP orBAlq. See Baldo et al, Appl. Phys. Lett. 75 (1999) 4 and Kwong et al,Appl. Phys. Lett. 81 (2002) 162. Skipping the ETL2 layer, i.e.anode/CuPc/α-NPD/CBP:Ir(ppy)₃/Alq₃/cathode, generates less than half theefficiency of the those with BCP or BAlq as the ETL2.

This is believed to be due to the unbalanced hole and electron transportin the EML because both CBP and Ir(ppy)₃ are more hole transporting thanelectron transporting. Moreover, Alq₃ does not inject electrons wellinto CBP and mCP, presumably due to the electron injection barrier andthe dipole moment of Alq₃ (reference low dipole application). As aresult, during operation, it is believed that holes migrate fastertowards the ETL side faster than electrons migrate to the HTL side. Toachieve good efficiency in this unbalanced device, BCP is used as anETL2 because it has a deep HOMO level (−5.87 eV) which effectivelyblocks holes from migrating into the ETL1 or BAlq is used as the ETL2because it can inject electrons well into the EML and has betterblocking properties than Alq₃. FIG. 4 illustrates this point.

The device structure is ITO/CuPc (10 nm)/α-NPD (30 nm)/CBP:Ir(5-Phppy)₃(30 nm and 12%)/2,3,6,7,10,11-hexphenyltriphenylene (5 nm)/Alq₃ (45nm)/LiF/Al where 2,3,6,7,10,11-hexphenyltriphenylene has a deep HOMO of−5.54 eV and a triplet energy of 2.6 eV which therefore facilitatesblocking at the EML/ETL2 interface. The molecule2,3,6,7,10,11-hexphenyltriphenylene is:

FIG. 4 shows the EL spectrum of this device and 2 similar devices where2 nm of 0.5% rubrene have been co-doped into the EML along with CBP andIr(5-Phppy)₃ at 3 respective places:

-   -   1. the HTL/EML interface i.e. ITO/CuPc (10 nm)/α-NPD (30        nm)/CBP:Ir(5-Phppy)₃ (12%):Rubrene (0.5%)(2 nm)/CBP:Ir(5-Phppy)₃        (28 nm and 12%)/HPT (5 nm)/Alq₃ (45 nm)/LiF/Al    -   2. in the middle of the EML i.e. ITO/CuPc (10 nm)/α-NPD (30        nm)/CBP:Ir(5-Phppy)₃ (14 nm and 12%)/CBP:Ir(5-Phppy)₃        (12%):Rubrene (0.5%)(2 nm)/CBP:Ir(5-Phppy)₃ (14 nm and 12%)/HPT        (5 nm)/Alq₃ (45 nm)/LiF/Al    -   3. at the EML/ETL interface i.e. ITO/CuPc (10 nm)/α-NPD (30        nm)/CBP:Ir(5-Phppy)₃ (28 nm and 12%)/CBP:Ir(5-Phppy)₃        (12%):Rubrene (0.5%)(2 nm)/HPT (5 nm)/Alq₃ (45 nm)/LiF/Al

Ir(5-Phppy)₃ is a green phosphorescent dopant with a peak emissionintensity at 518-520 nm. Rubrene has a characteristic yellow/orangecolor with a peak emission intensity at 558 nm. When rubrene is co-dopedin small amounts into the green phosphorescent OLED it can emit viaphosphorescent sensitized fluorescence (B. D'Andrade et al. Appl. Phys.Lett. (2001) 79-7, p. 1045) if there are green excitons sufficientlyclose enough to the rubrene that can transfer their energy to therubrene dopant. The location of the rubrene ‘probe’ layer therefore actsas an indicator of the amount of exciton recombination at a particularplace within the EML. FIG. 4 shows that the majority of the excitonrecombination in the CBP:Ir(5-Phppy)₃/HPT device occurs at the EML/ETL2interface.

The probe method used herein enables a determination to be made aboutwhere recombination occurs in the phosphorescent OLED. By way ofexample, a first OLED is procured which includes an emissive layer, theemissive layer comprising a host material and a first phosphorescentdopant material. The phosphorescent dopant material emits light having afirst spectra when electric current is applied to the OLED. A test OLEDis formed having a structure substantially identical to that of thefirst OLED, and further includes a test dopant in a first region of theemissive layer. A “substantially identical” structure allows for normalmanufacturing variation. Excitons preferentially transfer from theemissive material to the test dopant, and the test dopant has anemissive spectra different from that of the emissive material. Bymeasuring and comparing the light output of the first OLED and the testOLED, when the first OLED and the test OLED are operated under similarconditions, a determination is made as to whether the test dopant is ata region of the test OLED from which the emissive material would emitlight if the test dopant were not present. “Operated under similarconditions” means similar current and/or voltage. The measuring andcomparing is preferably a comparison of the spectra of the two devices,but could include measurements such as luminous output or peakwavelength.

While the examples herein use rubrene, the method is not so limited.Preferably, the test dopant is a fluorescent material, although it couldinstead be a second phosphorescent material. Preferably, the test dopantworks by phosphorescent sensitized fluorescence, and the test dopantpreferably does not affect or interfere with device operation, otherthan changing the electroluminescent spectrum. The test dopant may havean emission spectra that is lower in energy than that of the firstphosphorescent dopant material. Preferably, a density of the test dopantin the localized area is between 0.5% and 0.01%. A test dopant that doesnot emit would be considered to have an emissive spectra “different”from that of the emissive material.

The testing method can be expanded to form a plurality of test OLEDs,each test OLED having a structure substantially similar to the firstOLED, and each test OLED including the test dopant disposed in adifferent region of the emissive layer. The light output from each ofthe plurality of test OLEDs is measured and compared to the light outputof the first OLED to determine from which region of the first OLED lightis being emitted.

FIG. 5 shows the electroluminescent spectrum of the same devicestructure with rubrene fluorescent probes inserted, as in FIG. 4. Theonly difference is the green phosphorescent dopant is 6% Ir(3′-Meppy)₃instead of 12% Ir(5-Phppy)₃. In case of Ir(3′-Meppy)₃ dopant themajority of recombination in the EML occurs next to HTL interface (theintensity of the rubrene peak is the highest when the probe is insertednext to HTL).

This means that Ir(3′-Meppy)₃ dopant is less hole transporting and moreelectron transporting than Ir(5-Phppy)₃ (compare HOMO and LUMO levels ofIr(5-Phppy)₃ versus Ir(3′-Meppy)₃; they have the same HOMO, butIr(3′-Meppy)₃ has a 0.3 eV higher LUMO. We believe this shows that theHOMO/LUMO levels do not correlate to charge mobility that much. Rather,the levels correlate more to the barrier to charge injection). For anon-blocked architecture the location of recombination zone is critical.The further away the recombination occurs from ETL interface, the lesssensitive should be the device efficiency to the blocking layer (ETL2)layer.

Low dipole materials such as Zrq₄ and Hfq₄ may inject electrons wellinto the EML. For its explanation of low dipole materials and foradditional low dipole materials that may be used, U.S. PublishedApplication 20040197601 A1, published Oct. 7, 2004, is incorporatedherein by reference. To simplify the device structure hence reducingmanufacturing cost, it is preferred to eliminate the use of two ETLs andonly use a single ETL e.g. Alq₃ directly on top of the EML.α-NPD/Host:Ir(ppy)₃/Alq₃ may have good efficiencies. See Adachi et al.,“Architectures For Efficient Electrophosphorescent OrganicLight-Emitting Devices,” IEEE J. on Selected Topics in Quantum Elec., 8,372-377 (2002); and Adachi et al., “Nearly 100% Internal PhosphorescentEfficiency In An Organic Light Emitting Device,” J. Appl. Phys., 90,5048 (2001). In these cases, the mechanism is believed to be the hostssuch as BCP (HOMO=−5.87 eV, HOMO ΔE=1.16 eV with respect to α-NPD) withdeep HOMO severely limit the injection and migration of holes. Thesedevices also show poor stability and are therefore not practical incommercial applications. For example in T. Watanabe, K. Nakamura, S.Kawami, Y. Fukuda, T. Tsuji, T. Wakimoto, S. Miyaguchi, Proceedings ofSPIE 4105 (2000) 175 and T. Watanabe, K. Nakamura, S. Kawami, Y. Fukuda,T. Tsuji, T. Wakimoto, S. Miyaguchi, M. Yahiro, N.-J. Yang, T. Tsutsui,Synthetic Metals 1221 (2001) 203, the instability of BCP containingdevices is demonstrated, with the cause suspected to be electrochemicaland morphological instabilities.

Based on our research on recombination with the rubrene fluorescentprobes, we developed an organic light emitting diode (OLED) architecturein which efficient operation is achieved, in some examples, inwavelengths corresponding to green without requiring a blocking layer bylocating the recombination zone close to the hole transport side of theemissive layer. The concepts are applicable to all wavelengths. Thechoice of host and phosphorescent dopant materials can effect thelocation of the recombination zone within the emissive layer of thedevice. Based on the hole or electron transporting ability of thematerial due to HOMO-LUMO energy levels position and/or hole or electronmobility, we were able to find host and phosphorescent dopantcombinations which allowed building of devices with recombination zonesshifted to the hole transport layer side of the emissive layer, therebyproviding an efficient structure without a blocking layer. Thissimplifies device structure by eliminating the blocking layer.

As an embodiment of the present invention, a device structure isdesigned such that the hosts can accept electrons readily from theelectron transport layer such as Alq₃, offering efficient operation withno requirement for an additional electron transport layer to facilitatehole blocking or electron injection. The LUMO level of such hosts isdesigned to be lower than those of commonly used hosts such as CBP ormCP, while retaining effective hole injection from the HTL and holemigration in the EML to achieve good hole-electron balance, and hightriplet energy levels. As is understood in the art, the π-conjugation ofaromatic systems may be increased by extending the π-conjugation byfusing aryl rings (for example, using napthalene or phenanthrene versusbenzene or biphenyl) or extending the double/triple bonds by e.g. orthoor para substitutions (for example, using stilbene versus biphenyl). Thedegree of π-conjugation affects the HOMO and LUMO properties ofcompounds. Generally, increasing the degree of π-conjugation alsodecreases the band gap by raising the HOMO level and/or lowering theLUMO level which are both desirable from the perspective of chargeinjection and migration. Embodiments of the invention are believed topossess sufficiently high triplet energy levels for use in non-redOLEDs. It is also believed that the oxidized (cation radical) andreduced (anion radical) states of organic materials with high degree ofπ-conjugation have higher stability than the less conjugated ones. Thismay be because in the charged state the hole or electron can delocalizemore extensively.

FIGS. 6 through 10 show that the new 2,7-DCP host material, designedwith above properties, has superior electron transporting properties tothose of CBP. As evidence, FIG. 7 shows EL spectra of CuPc (100 Å)/NPD(400 Å)/X/Alq₃ (500 Å)/LiF/Al device with X equal to a 100 Å of CBP or2,7-DCP layer (FIG. 6, layer 610) at the interface between NPD and Alq₃,as shown in FIG. 6. With 2,7-DCP layer we can see some NPD emission inthe spectrum, whereas with CBP the emission is solely from Alq₃ (Alq₃has a peak emission wavelength of ˜520 nm and NPD has a peak emissionwavelength of ˜430 nm). It indicates that DCP possess better electrontransporting and hole blocking properties than CBP (i.e., can transportelectrons to NPD layer for recombination there and block some holes fromgetting to Alq₃ and recombining there).

FIG. 9 shows the current density versus voltage and FIG. 10 shows theexternal quantum efficiency versus current density of aCuPc/NPD/Alq₃/X/Alq₃/LiF/Al device, where X=CBP or 2,7-DCP 100 Å layer(FIG. 8, layer 810) inserted into the Alq₃ ETL, as shown in FIG. 8. The2,7-DCP layer device has lower voltage and significantly higherefficiency than the device with CBP layer in ETL. It supports thestatement that 2,7-DCP is a better electron transporting material thanCBP.

External quantum efficiency as used herein refers to photons out perelectrons in for devices that have approximately 20 to 30% outcoupling.

Other example host materials such as those included below in Table 1 maybe used to achieve similar results. Preferably, the host is a fused-arylring. Specific examples of a fused-aryl ring include a carbazole or adicarbazole. Additional examples of carbazoles may be found in U.S.application Ser. No. 10/971,844 entitled “Arylcarbazoles As Hosts InPhOLEDs” filed Oct. 22, 2004, the examples from which are incorporatedherein by reference. Preferably, the host is combined with an organicdopant having a triplet energy corresponding to a peak emissionwavelength of less than 600 nm. For example, the dopant may be selectedfrom phosphorescent materials such as Ir(5-Phppy)₃ or Ir(3′-Meppy)₃.

Further refinement to position the triplet recombination zone at theinterface of the emissive layer with the hole transport layer may beexperimentally achieved by adjusting layer thicknesses, materials, andhost/dopant ratios, using for example the probe method referred toabove. By adjusting values and forming test devices including a probelayer, the position of recombination may be determined. Once a desiredposition is achieved, devices can be produced using these values, butomitting the probe layer. As used herein the recombination zone “at theinterface of the emissive layer with the HTL” means in the emissivelayer, but with peak recombination positioned not more than 10% of thethickness of the emissive layer away from the interface.

A schematic energy level diagram of a phosphorescent OLED withoutblocking layers is illustrated in FIG. 11. For each individual layer,the HOMO level of the layer material is responsible for hole transportthrough the layer and the LUMO level is responsible for electrontransport through this layer. For the emissive layer consisting of hostand dopant components, HOMO and LUMO levels of both the host and dopantcan transport charge through the layer. The relative position of hostand dopant HOMO and LUMO levels defines which component transports holesand electrons through the emissive layer. As is apparent from FIG. 11:Eg1 is the energy gap of the dopant; Eg2 is the energy gap of the host;ΔE1 is the energy difference between the HOMO of the HTL and the HOMO ofthe host; ΔE2 is the energy difference between the HOMO of the HTL andthe HOMO of the dopant; ΔE3 is the energy difference between the LUMO ofthe ETL and the LUMO of the dopant; ΔE4 is the energy difference betweenthe LUMO of the ETL and the LUMO of the host; ΔE5 is the energydifference between the HOMO of the host and the HOMO of the ETL; ΔE6 isthe energy difference between the HOMO of the dopant and the HOMO of theETL; ΔE7 is the energy difference between the LUMO of the HTL and theLUMO of the dopant, and ΔE8 is the energy difference between the LUMO ofthe HTL and the LUMO of the host.

As described above, typically a blocking layer is required between theemissive layer and ETL to confine the excitons and charges within theemissive layer and prevent holes and exciton leakage to the ETL, inorder to maintain a high device efficiency and a pure emission spectrum.Due to higher hole mobility as compared to electron mobility, theblocking layer is much more important on the ETL side rather than theHTL side of the emissive layer. However, if the recombination zone islocated close to the HTL side of the emissive layer (away from the ETLinterface) the blocking layer on the ETL side may not be necessary,since the majority of excitons are formed at the emissive layer-HTLinterface. The ability of host (or dopant) to transport electrons ratherthan holes can shift the recombination zone away from the ETL interface,and then may allow no blocking layer phosphorescent OLED structures withhigh efficiency.

Based on our experimental results and referring to FIG. 11, we concludethat an efficient device can be realized by selecting a combination ofhole transport material, host material, and electron transport materialresulting in a ΔE₁≦0.8 eV, ΔE₄≦0.4 eV, and ΔE₅≦0.4 eV. As a furtherembodiment, ΔE₁/ΔE₄<2.0 in order to balance carrier transportcharacteristics of the carrier types. The organic dopant is preferably aphosphorescent material, having a triplet energy corresponding to a peakemission wavelength of less than 600 nm. Example dopants includeIr(5-Phppy)₃ and Ir(3′-Meppy)₃. These characteristics produce anefficient device. Further refinement to position the tripletrecombination zone at the interface of the emissive layer with the holetransport layer may be experimentally achieved by adjusting layerthicknesses, materials, and host/dopant ratios, using for example theprobe method described above.

Density functional calculations (DFT) of HOMO and LUMO levels arepreferred over literature calculations due to consistency from iterationto iteration, whereas literature values can show significant energylevel variations for the same energy level of a same material based uponthe equipment used to measure the value. As computed herein, allHOMO-LUMO density functional calculations were performed using theSpartan 02 software package, available from Wavefunction Inc. of Irvine,Calif., at the B3LYP/6-31G* level with the exception of DTBD, which wasperformed at the B3LYP/6-31G(d) level, and Ir(F₂CNppy)₂(Pic), which wasperformed at the B3LYP/CEP-31g level, both using the Gaussian98 softwarepackage.

The wavelengths of light to be emitted from the emissive layer is amaterial characteristic that depends upon the triplet energies, whichare characteristics of the materials selected for the emissive layer.The correlation between the actual triplet energy (actual; not densityfunctional calculations) and wavelength can be determined as follows:

$E_{Triplet} = {\left( \frac{6.626 \times 10^{- 34}J}{s} \right)\left( \frac{1{eV}}{1.602 \times 10^{- 19}J} \right)\left( \frac{2.998 \times 10^{8}m}{s} \right)\left( \frac{1}{\lambda} \right)}$

It is understood that the various embodiments described herein are byway of example only, and are not intended to limit the scope of theinvention. For example, many of the materials and structures describedherein may be substituted with other materials and structures withoutdeviating from the spirit of the invention. It is understood thatvarious theories as to why the invention works are not intended to belimiting. For example, theories relating to charge transfer are notintended to be limiting.

MATERIAL DEFINITIONS

As used herein, abbreviations refer to materials as follows:

-   CBP: 4,4′-N,N-dicarbazole-biphenyl-   m-MTDATA 4,4′,4″-tris(3-methylphenylphenlyamino)triphenylamine-   Alq₃: 8-tris-hydroxyquinoline aluminum-   Bphen: 4,7-diphenyl-1,10-phenanthroline-   n-BPhen: n-doped BPhen (doped with lithium)-   F₄-TCNQ: tetrafluoro-tetracyano-quinodimethane-   p-MTDATA: p-doped m-MTDATA (doped with F₄-TCNQ)-   Ir(ppy)₃: tris(2-phenylpyridine)-iridium-   Ir(ppz)₃: tris(1-phenylpyrazoloto,N,C(2′)iridium(III)-   Ir(3′-Meppy)₃: iridium(III) tris(2-phenyl-3-methylpyridine)-   Ir(5-Phppy)₃: iridium(III) tris(2-(3-biphenyl)pyridine)-   BCP: 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline-   2,7-DCP 2,7-N,N-dicarbazolephenanthren-   2,7-DCPT: 2,7-di(4-(N-carbazole)phenyl)triphenylene-   TAZ: 3-phenyl-4-(1′-naphthyl)-5-phenyl-1,2,4-triazole-   CuPc: copper phthalocyanine.-   ITO: indium tin oxide-   HPT: 2,3,6,7,10,11-hexaphenyltriphenylene-   NPD: N,N′-diphenyl-N—N′-di(1-naphthyl)-benzidine-   TPD: N,N′-diphenyl-N—N′-di(3-toly)-benzidine-   BAlq:    aluminum(III)bis(2-methyl-8-hydroxyquinolinato)-4-phenylphenolate-   mCP: 1,3-N,N-dicarbazole-benzene-   DCM: 4-(dicyanoethylene)-6-(4-dimethylaminostyryl-2-methyl)-4H-pyran-   DMQA: N,N′-dimethylquinacridone-   PEDOT:PSS: an aqueous dispersion of poly(3,4-ethylenedioxythiophene)    with polystyrenesulfonate (PSS)

Table 1 illustrates example host materials and properties calculatedusing DFT: HOMO LUMO (eV) (eV) CBP

−5.32 −1.23 2,7-DCPT

−5.3 −1.57 2,7-DCT

−5.28 −1.36 3,6-DCT

−5.32 −1.35 2,6,10-TCT

−5.42 −1.5 2,7-DCP

−5.3 −1.42 3,6-DCP

−5.28 −1.44 2,6-DCN

−5.3 −1.52 LUMO- Dipole HOMO ΔE1 (w.r.t. α- ΔE4 (w.r.t. ΔE5 (w.r.t.(debye) (eV) NPD) Alq₃) Alq₃) CBP 0 4.09 0.61 0.5 −0.32 2,7-DCPT 0.133.73 0.59 0.16 −0.3 2,7-DCT 0.37 3.92 0.57 0.37 −0.28 3,6-DCT 2.89 3.970.61 0.38 −0.32 2,6,10-TCT 0.15 3.92 0.71 0.23 −0.42 2,7-DCP 0.03 3.880.59 0.31 −0.3 3,6-DCP 2.57 3.84 0.57 0.29 −0.28 2,6-DCN 0 3.78 0.590.21 −0.3

EXPERIMENTAL

All devices are fabricated in high vacuum (<10⁻⁷ Torr) by thermalevaporation. The anode electrode is ˜1200 Å of indium tin oxide (ITO).The cathode consists of 10 Å of LiF followed by 1,000 Å of Al. Alldevices are encapsulated with a glass lid sealed with an epoxy resin ina nitrogen glove box (<1 ppm of H₂O and O₂) immediately afterfabrication, and a moisture getter was incorporated inside the package.Specific representative embodiments of the invention will now bedescribed and compared to prior art examples. It is understood that thespecific methods, materials, conditions, process parameters, apparatusand the like do not necessarily limit the scope of the invention.

In the experimental examples, the dopant used is Ir(3′Meppy)₃. TheIr(3′Meppy)₃ molecule is:

Comparisons were performed between CBP and 2,7-DCP hosts in aphosphorescent OLED without a blocking layer having the structureillustrated in FIG. 12. The specific test structures were ITO (1200Å)/HIL (100 Å)/NPD (300 Å)/host:dopant (4.5% 300 Å)/Alq₃ (450 Å)/LiF (10Å)/Al (1000 Å). The results are illustrated in Table 2:

Example 1 Example 2 Example 3 Example 4 Host DCP DCP CBP CBP Dopant Ir(3′-Meppy)₃ Ir(3′-Meppy)₃ Ir(3′-Meppy)₃ Ir(3′-Meppy)₃ HIL CuPcIr(3′-Meppy)₃ CuPc Ir(3′-Meppy)₃ Color green green green green CIE x0.32 0.33 0.32 0.33 CIE y 0.63 0.62 0.63 0.62 External quantum 9.2 10.27.2 7.8 efficiency at 1000 cd/m², % Luminous efficiency 34.1 37.8 26.728.8 at 1000 cd/m², cd/A Driving voltage at 8.2 6.8 9.2 8.0 1000 cd/m²,V Power efficiency at 13.1 17.5 9.1 11.4 1000 cd/m², lm/W 50% luminancedrop of 370 hours 330 hours the device DC driven at room temperature, 40mA/cm²

Further results for the four examples are illustrated in FIGS. 13through 17. FIG. 13 illustrates the normalized electroluminescencespectra at 10 mA/cm²; FIG. 14 illustrates the luminous efficiency versusluminance; FIG. 15 illustrates the power efficiency versus theluminance; FIG. 16 illustrates the current density versus voltage; andFIG. 17 illustrates the luminance versus voltage.

FIG. 18 illustrates the device room temperature lifetime at a constantdc current density of 40 mA/cm² for the non-blocked green structures inexamples 1 and 3 having the 2,7-DCP:Ir(3′-Meppy)₃ or CBP:Ir(3′-Meppy)₃emissive layer. FIG. 19 illustrates the room temperature device lifetimeat a constant dc current density from an initial luminance of 1000 cd/m²for the non-blocked green structures in example 1 having the2,7-DCP:Ir(3′-Meppy)₃ emissive layer. The dashed portion of the plot inFIG. 19 is extrapolated from the measured data points. These figuresdemonstrate that the non blocked green phosphorescent OLED device hasgood operational lifetime.

As demonstrated in FIG. 19, for a non-blocked green structures,embodiments of the invention may achieve commercially acceptablelifetimes, i.e., >3,000 hours to 80% of initial display luminance of1,000 cd/m² (measured), or >10,000 hours to 50% of a initial displayluminance of 1,000 cd/m² (extrapolated).

Although Ir(3′-Meppy)₃ is used in these examples, it is believed thatother phosphorescent materials such as Ir(5-Phppy)₃ would achievesimilar results. Likewise, other hosts such as those presented in Table1 may be used.

As demonstrated by these results, DCP is a better electron-transportinghost than the conventional CBP. Accordingly, non-blocking structureswith a DCP host are about 30% more efficient as compared to their CBPhost analogs.

Further refinement of these examples to position the tripletrecombination zone at the interface of the emissive layer with the holetransport layer may be experimentally achieved by adjusting layerthicknesses, materials, and host/dopant ratios, using for example theprobe method described above.

The simple layered structure illustrated in FIGS. 1, 2, 6, 8, 11, and12, and are provided by way of non-limiting example, and it isunderstood that embodiments of the invention may be used in connectionwith a wide variety of other structures. The specific materials andstructures described are exemplary in nature, and other materials andstructures may be used. Functional OLEDs may be achieved by combiningthe various layers described in different ways, or layers may be omittedentirely, based on design, performance, and cost factors. Other layersnot specifically described may also be included. Materials other thanthose specifically described may be used. Although many of the examplesprovided herein describe various layers as comprising a single material,it is understood that combinations of materials, such as a mixture ofhost and dopant, or more generally a mixture, may be used. Also, thelayers may have various sublayers. The names given to the various layersherein are not intended to be strictly limiting. For example, in device200, hole transport layer 225 transports holes and injects holes intoemissive layer 220, and may be described as a hole transport layer or ahole injection layer. In one embodiment, an OLED may be described ashaving an “organic layer” disposed between a cathode and an anode. Thisorganic layer may comprise a single layer, or may further comprisemultiple layers of different organic materials as described, forexample, with respect to FIGS. 1, 2, 6, 8, 11, and 12.

Structures and materials not specifically described may also be used,such as OLEDs comprised of polymeric materials (PLEDs) such as disclosedin U.S. Pat. No. 5,247,190, Friend et al., which is incorporated byreference in its entirety. By way of further example, OLEDs having asingle organic layer may be used. OLEDs may be stacked, for example asdescribed in U.S. Pat. No. 5,707,745 to Forrest et al, which isincorporated by reference in its entirety. The OLED structure maydeviate from the simple layered structure illustrated in FIGS. 1 and 2.For example, the substrate may include an angled reflective surface toimprove outcoupling, such as a mesa structure as described in U.S. Pat.No. 6,091,195 to Forrest et al., and/or a pit structure as described inU.S. Pat. No. 5,834,893 to Bulovic et al., which are incorporated byreference in their entireties.

Unless otherwise specified, any of the layers of the various embodimentsmay be deposited by any suitable method. For the organic layers,preferred methods include thermal evaporation, ink-jet, such asdescribed in U.S. Pat. Nos. 6,013,982 and 6,087,196, which areincorporated by reference in their entireties, organic vapor phasedeposition (OVPD), such as described in U.S. Pat. No. 6,337,102 toForrest et al., which is incorporated by reference in its entirety, anddeposition by organic vapor jet printing (OVJP), such as described inU.S. patent application Ser. No. 10/233,470, which is incorporated byreference in its entirety. Other suitable deposition methods includespin coating and other solution based processes. Solution basedprocesses are preferably carried out in nitrogen or an inert atmosphere.For the other layers, preferred methods include thermal evaporation.Preferred patterning methods include deposition through a mask, coldwelding such as described in U.S. Pat. Nos. 6,294,398 and 6,468,819,which are incorporated by reference in their entireties, and patterningassociated with some of the deposition methods such as ink-jet and OVJD.Other methods may also be used. The materials to be deposited may bemodified to make them compatible with a particular deposition method.For example, substituents such as alkyl and aryl groups, branched orunbranched, and preferably containing at least 3 carbons, may be used insmall molecules to enhance their ability to undergo solution processing.Substituents having 20 carbons or more may be used, and 3-20 carbons isa preferred range. Materials with asymmetric structures may have bettersolution processibility than those having symmetric structures, becauseasymmetric materials may have a lower tendency to recrystallize.Dendrimer substituents may be used to enhance the ability of smallmolecules to undergo solution processing.

The molecules disclosed herein may be substituted in a number ofdifferent ways without departing from the scope of the invention. Forexample, substituents may be added to a compound having three bidentateligands, such that after the substituents are added, one or more of thebidentate ligands are linked together to form, for example, atetradentate or hexadentate ligand. Other such linkages may be formed.It is believed that this type of linking may increase stability relativeto a similar compound without linking, due to what is generallyunderstood in the art as a “chelating effect.”

Devices fabricated in accordance with embodiments of the invention maybe incorporated into a wide variety of consumer products, including flatpanel displays, computer monitors, televisions, billboards, lights forinterior or exterior illumination and/or signaling, heads up displays,fully transparent displays, flexible displays, laser printers,telephones, cell phones, personal digital assistants (PDAs), laptopcomputers, digital cameras, camcorders, viewfinders, micro-displays,vehicles, a large area wall, theater or stadium screen, or a sign.Various control mechanisms may be used to control devices fabricated inaccordance with the present invention, including passive matrix andactive matrix. Many of the devices are intended for use in a temperaturerange comfortable to humans, such as 18 degrees C. to 30 degrees C., andmore preferably at room temperature (20-25 degrees C.).

The materials and structures described herein may have applications indevices other than OLEDs. For example, other optoelectronic devices suchas organic solar cells and organic photodetectors may employ thematerials and structures. More generally, organic devices, such asorganic transistors, may employ the materials and structures.

While the present invention is described with respect to particularexamples and preferred embodiments, it is understood that the presentinvention is not limited to these examples and embodiments. The presentinvention as claimed therefore includes variations from the particularexamples and preferred embodiments described herein, as will be apparentto one of skill in the art.

1. A device comprising: an anode and a cathode; an emissive layerdisposed between and electrically connected to the anode and thecathode, the emissive layer comprising an organic host and an organicdopant; an organic hole transport layer between the anode and theemissive layer; an organic electron transport layer between the cathodeand the emissive layer, wherein said electron transport layer is asingle layer and is the only layer between the cathode and the emissivelayer, the HOMO level of the organic host is no more than 0.8 eV belowthe HOMO level of the hole transport layer, the LUMO level of theorganic host is no more than 0.4 eV above the LUMO level of the electrontransport layer, the HOMO level of the electron transport layer is nomore than 0.4 eV below the HOMO level of the host, the HOMO and LUMOlevels being calculated by density functional calculation, and whereinthere is a triplet recombination zone within the emissive layer not morethan 10% of the thickness of the emissive layer away from an interfacebetween the emissive layer with the hole transport layer.
 2. The deviceof claim 1, wherein the organic dopant is a phosphorescent material. 3.The device of claim 2, wherein the organic dopant has a triplet energycorresponding to a peak emission wavelength of less than 600 nm.
 4. Thedevice of claim 3, wherein the dopant is an iridium(III) organometalliccomplex.
 5. The device of claim 2, wherein the dopant is Ir(5-Phppy)₃.6. The device of claim 2, wherein the dopant is Ir(3′-Meppy)₃.
 7. Thedevice of claim 2, wherein$\frac{{HOMO}_{Host} - {HOMO}_{HoleTransportLayer}}{{LUMO}_{ElectronTransportLayer} - {LUMO}_{Host}} < {2.0.}$8. A device comprising: an anode and a cathode; an emissive layerdisposed between and electrically connected to the anode and thecathode, the emissive layer comprising an organic host and an organicdopant; an organic hole transport layer between the anode and theemissive layer; an organic electron transport layer between the cathodeand the emissive layer, wherein said electron transport layer is asingle layer and is the only layer between the cathode and the emissivelayer, the organic dopant is a phosphorescent dopant having a tripletenergy corresponding to a peak emission wavelength of less than 600 nm,the host comprises a fused-aryl ring compound; and wherein there is atriplet recombination zone within the emissive layer, not more than 10%of the thickness of the emissive layer away from an interface betweenthe emissive layer with the hole transport layer.
 9. The device of claim8, wherein the fused-aryl ring compound comprises a naphthalene moiety.10. The device of claim 8, wherein the fused-aryl ring compoundcomprises a phenanthrene moiety.
 11. The device of claim 8, wherein theorganic host is a dicarbazolephenanthren compound.
 12. The device ofclaim 8, wherein the organic dopant is Ir(5-Phppy)₃.
 13. The device ofclaim 9, wherein the organic dopant is Ir(3′-Meppy)₃.
 14. The device ofclaim 8, wherein the organic host is a dicarbazolephenanthren compoundand the organic dopant is Ir(3′-Meppy)₃.
 15. A device comprising: ananode and a cathode; an emissive layer disposed between and electricallyconnected to the anode and the cathode, the emissive layer comprising anorganic host and an organic dopant; an organic hole transport layerbetween the anode and the emissive layer; an organic electron transportlayer between the cathode and the emissive layer, wherein said electrontransport layer is a single layer and is the only layer between thecathode and the emissive layer, the organic dopant is a phosphorescentdopant having a triplet energy corresponding to a peak emissionwavelength of less than 600 nm, and wherein there is a tripletrecombination zone within the emissive layer not more than 10% of thethickness of the emissive layer away from an interface between theemissive layer with the hole transport layer.
 16. The device of claim15, wherein the dopant is Ir(5-Phppy)₃.
 17. The device of claim 15,wherein the dopant is Ir(3′-Meppy)₃.
 18. The device of claim 15, whereinthe device has a lifetime>3,000 hours to 80% of an initial luminance of1,000 cd/m².
 19. The device of claim 15, wherein the device has alifetime>10,000 hours to 50% of an initial luminance of 1,000 cd/m².